Oxygen enriched air and pure oxygen have been utilized as an oxidizing gas for combustion purposes in furnaces for some time. Partial or complete substitution of oxygen for air and other oxidizing gases in combustion processes has resulted in a substantial decrease in flue gas volumes as well as a significant increase in flame adiabatic temperatures and the percent of heat available for heating processes. On the other hand, the utilization of a highly concentrated oxidizing gas, in many case, has caused a rapid deterioration of furnace components, increased NOx emissions and localized overheating of the load and furnace lining.
To overcome some of the above limitations of prior art combustion systems, U.S. Pat. No. 4,378,205 describes a method and apparatus utilizing a step of injecting into the furnace atmosphere at least one jet of an oxidant gas with high oxygen content by volume which has the necessary velocity and direction to aspirate the furnace gases from the vicinity about the oxidant jet into the oxidant jet, and further to mix the oxidant jet with a fuel jet inside of the furnace interior to form the flame. The aspiration of the furnace gases into the flame in this manner tends to cause the furnace gases to be reburned in the flame.
The described open stream oxidant gas method tends to reduce the adiabatic temperature of the oxygen enriched flame formed in the furnace atmosphere and provides for dilution of the flame generated inside the furnace atmosphere with combustion products aspirated from the furnace chamber by the oxidant gas which travels inside of the furnace atmosphere prior to participation in combustion. Delaying the mixing of the gases between the fuel jets and the oxidant gas jets in addition to dilution of the oxidant jet within the furnace atmosphere causes the flame to have a low luminosity, diminishes the heat release density at the burner nozzle and reduces NOx formation by eliminating the high temperature flame core. This combustion method uses furnace space to accomplish the dilution of at least a part of the oxidizing gas with furnace gases prior to the oxidizing gas becoming involved with the fuel. Also this combustion method has the capability to reduce NOx formation and to decrease adiabatic flame temperatures but has substantial limitations.
First, this open stream oxidant gas method requires a substantial part of the working volume of the furnace combustion chamber to be empty for the mixing of the fuel and oxidant gas to develop the flame within the working volume of the combustion chamber of the furnace before the flame engages in the heating of the load or work product in the combustion chamber. Any contact of the oxidant gas with the surface of the load prior to mixture with the fuel would accelerate oxidation of the heated products. The low heating density specifically in the area surrounding the burner necessitates an increase in the furnace working volume for flame development and therefore increases the capital cost of the heating equipment.
The open combustion approach does not utilize a burner combustion tunnel for fuel and oxidant gas mixing to provide flame stability and flame velocity through the shaping of the hot expanded combustion gasses generated in a burner tunnel prior to discharging into the furnace atmosphere. The absence of a burner combustion tunnel makes the flame substantially less stable and therefore reduces flame velocity and therefore reduces the rate of convective heat transfer from the flame to the load. The convective heat exchange from combustion products to the load is desirable to insure rapid and uniform heating of the load.
There exists, therefore, a need for an aspirating combustion system and method which utilizes a highly oxygen-concentrated oxidizing gas which results in more efficient heating of the load with a flame in industrial furnaces and results in reburning inside the combustion chamber of the burner some of the gases of the furnace atmosphere which reduces the products of incomplete combustion and air borne contaminants in the flue gas.
There also exists a need for a method of heating within an industrial furnace which results in maximization of the furnace throughput, keeping adequate uniformity of heating.
There exists a still further need for a combustion system and method which can utilize a highly oxygen-concentrated oxidizing gas to provide a high velocity flame and a high temperature turbulence furnace atmosphere having a low NOx content in the flue gases.